The present invention relates to a process for forming a magnetic thin film on a thin plastic base film such as a polyester film, a heat treatment process of a sheet of the base film and the magnetic thin film, and an apparatus for performing heat treatment therefor.
A magnetic tape of a metal thin film type has received great attention as a high density recording medium. Various processes for fabricating the magnetic tape of the metal thin film type have been proposed. Among these processes, typical examples are vacuum deposition, electron beam deposition, sputtering, and ion-plating. A magnetic tape which is fabricated by one of these processes has an extremely thin magnetic layer as compared with a conventional magnetic tape on which magnetic particles together with a binder are coated on a base film. The thinner the magnetic layer is, the better the high frequency response is in recording and playback. For this reason, when the magnetic thin film is used, a magnetic recording tape which is suitable for high density recording is obtained.
The base film which is used in a metal thin film type magnetic recording tape for long time recording is extremely thin. Assuming that this thin film magnetic recording tape is fabricated by vacuum deposition, the base film is thermally damaged by radiant heat from a vapor source and latent heat of vaporized metal atoms. Then, wrinkles are formed on the film. Further, while the vaporized metal atoms are recrystalized and formed as a thin film, internal stress corresponding to heat shrinkage of the thin film occurs. A base film 10 is curled with a thin film layer 12 facing inward due to inner stress, as shown in FIG. 1A. When a magnetic recording tape on which a large wrinkle is formed is used, dropout occurs. Further, when a curled magnetic recording tape is used, it vertically deviates from the normal tape driving path (i.e., it meanders). Thus, the tape is misaligned with the head and the tape is irregularly wound around a take-up reel.
In general, a thin film magnetic recording tape is fabricated as follows. A magnetic material is deposited on a base film strip of a wide width. Various coatings and surface treatments are performed. A thin film sheet of a great width is cut into narrow bands of a predetermined width. Thus, the magnetic tape is fabricated. The width of the thin film sheet on which the magnetic particles are deposited is considerably greater than the magnetic recording tape as the final product. In order to completely eliminate wrinkles and curls from the thin film magnetic recording tape, the thin film sheet must be completely smoothed before it is cut into narrow bands.
For this purpose, the base film on which the thin film is formed may be heated. However, when the base film with the thin film thereon as shown in FIG. 1A is simply heated, the curled film 10 becomes corrugated, as shown in FIG. 1B. Therefore, with a simple heat treatment as described above, the wrinkles and curl cannot be uniformly eliminated from the entire surface of the film 10.
As the prior art which is proposed to eliminate the wrinkles and the curls, Japanese Patent Disclosure Nos. 53-83706 and 53-104204 disclose measures for this purpose. In these patent disclosures, cracks are formed in the magnetic particle layer to solve the above problem. However, formation of cracks entails another problem. Stiffness of the tape in the longitudinal direction of the tape becomes smaller than that in the direction of width thereof. When the magnetic recording tape is driven, the tape slidably driven past the magnetic head is not brought into contact with the magnetic head over its entire width. Thus, spacing loss is increased. Further, since stress concentrates especially in the cracks, resistance to oblique tearing of the magnetic recording tape may be considerably decreased. In other words, the tape tends to be torn obliquely.